1. Field of the Invention
The present invention relates to a radio-frequency power amplifier used in a mobile communication device or the like.
2. Description of the Related Art
A mobile phone system, such as EDGE (enhanced data GSM environment) or UMTS (universal mobile telecommunications system), has recently been the focus as the worldwide standard of next-generation mobile communications. In the mobile phone system, such as EDGE or UMTS, an output power control is required over a wide range and the high linearity is required in the characteristic of a modulation method adopted by the system. On the other hand, in recent years, since a mobile camera, game, a television tuner, and the like are mounted as application of a mobile phone, the power consumption of a mobile phone has significantly increased. Due to the above situation, it is strongly requested that a radio-frequency power amplifier, which comprises a radio-frequency transmission circuit, have high linearity and high efficiency at the same time.
In a mobile phone system according to the related art, the modulation control is performed in an orthogonal modulation method, and in order to realize the high linearity in the method, a radio-frequency power amplifier should operate as a class-A type amplifier. However, in the case of a class-A operation, the radio-frequency power amplifier continuously consumes a DC current, which does not improve the overall efficiency.
FIG. 8 illustrates a radio-frequency power amplifier, to which power is supplied from a power control regulator, disclosed in U.S. Pat. No. 6,701,138. Reference numeral 1 denotes a voltage control regulator block from which a voltage controlled by a control signal Vramp inputted through a control signal input terminal 10 is output. Reference numeral 2 denotes a radio-frequency power amplifier block formed by three-stage amplifying transistors including a first-stage amplifying transistor 6, a second-stage amplifying transistor 7, and a third-stage amplifying transistor 8. Predetermined fixed voltages are applied for a power source voltage Vcc1 of the first-stage amplifying transistor 6 and a bias voltage Vbias applied to a bias circuit 9 of the radio-frequency power amplifier block 2, respectively, and the same power source voltages output from the voltage control regulator block 1 are applied for a power source voltage Vcc2 of the second-stage amplifying transistor 7 and a power source voltage Vcc3 of the third-stage amplifying transistor 8. The relationship between the power source voltages Vcc2 and Vcc3 and the control signal Vramp has the following characteristic:Vcc2=Vcc3=α×Vramp+β  (Equation 1)
Here, α is a gain in a voltage control regulator and β is an offset amount.
The radio-frequency power amplifier is characterized in that both the high linearity and the high efficiency can be realized at the same time by making the third-stage amplifying transistor 8 always perform a switching operation, such as a class-E operation, for all levels of the output power.
In the radio-frequency power amplifier that performs the switching operation described above, the radio-frequency output power Pout of the radio-frequency power amplifier has the following characteristic:Pout(W)=(2×Vcc−Vsat)2/(8×Rload)  (Equation 2)
Here, it is defined that Vcc=Vcc2=Vcc3 and Rload is a load resistance of the radio-frequency power amplifier.
Furthermore, a radio-frequency output voltage Vout of the radio-frequency power amplifier is obtained from Equation 2 as follows:Vout(V)=g×(2×Vcc−Vsat)  (Equation 3)
Here, g is expressed as an integer.
In addition, from Equations 1 and 3, the relationship between the control signal Vramp and the radio-frequency output voltage Vout can be represented as follows:
                                                                        Vout                ⁢                                                                  ⁢                                  (                  V                  )                                            =                                                2                  ×                                      α                    ′                                    ×                  Vramp                                +                                  2                  ×                                      β                    ′                                                  +                Vsat                                                                                        =                                                G                  ×                  Vramp                                +                Vramp_offset                                                                        (                  Equation          ⁢                                          ⁢          4                )            
Here, α′, β′, G and Vramp_offset are expressed as an integer, respectively.
As can be seen from Equation 4, the radio-frequency output voltage Vout is proportional to the control signal Vramp.
Further, the radio-frequency power amplifier can be used in an EER (Envelope Elimination and Restoration) technique, which is a polar modulation technique disclosed in JP-T-2004-501527. Since details of the EER technique is disclosed in JP-T-2004-501527, the EER technique will be briefly described. In the EER technique, a modulation signal in, for example, a mobile phone is divided into an amplitude component and a phase component, and the phase component is input as a radio-frequency signal to a radio-frequency input terminal 11 of the radio-frequency power amplifier and the amplitude component is input to a power source voltage terminal. Thereby, a signal in which the amplitude component and the phase component are mixed is output to a radio-frequency output terminal 12 of the radio-frequency power amplifier. The reason is that, because the radio-frequency power amplifier performs a switching operation such as a class-E operation, the relevance of the amplitude component is very low but the relevance of the phase component is very high between the radio-frequency input terminal 11 of the radio-frequency power amplifier and the radio-frequency output terminal 12 of the radio-frequency power amplifier, and the relevance of the amplitude component is very high between the power source voltage terminal of the radio-frequency power amplifier and the radio-frequency output terminal 12 of the radio-frequency power amplifier.
As such, in the modulation control of a radio-frequency transmission circuit of a mobile phone or the like, the high linearity and the high efficiency are realized by the EER technique.
However, in the EER technique, since the amplitude component of the modulation signal is input from the control signal Vramp to be reproduced to the radio-frequency output voltage Vout of the radio-frequency power amplifier, the control signal Vramp and the radio-frequency output voltage Vout should be proportional to each other and it is required not to have an offset. If the two conditions are not satisfied, the precision of modulation is significantly lowered, which causes the communication quality to be deteriorated.
However, the linearity of the radio-frequency power amplifier shown in FIG. 8 cannot be maintained in a region where the radio-frequency output voltage Vout is low as shown in FIG. 9. In the non-linear region, the control signal Vramp and the radio-frequency output voltage Vout are not proportional to each other. Further, in the relationship between the control signal Vramp and the radio-frequency output voltage Vout, as expressed in Equation 4, the radio-frequency output voltage Vout has only an offset amount Vramp_offset with respect to the control signal Vramp. The non-linearity in a region, where the radio-frequency output voltage Vout is so low, or the offset component Vramp_offset with respect to the control signal Vramp causes the precision of modulation in the EER technique to be significantly lowered, which deteriorates the communication quality.